It is known for a rotor shaft of a rotor, composed of turbine wheel, compressor wheel and rotor shaft, of an exhaust-gas turbocharger to be rotatably mounted in rolling bearings. The rotor shaft bears the turbine wheel of an exhaust-gas-driven turbine in one end region thereof, and bears the compressor wheel of a compressor in the other end region thereof. The rotor and thus the rotor shaft of an exhaust-gas turbocharger runs, during operation in conjunction with an internal combustion engine, with variable, very high rotational speeds, and must withstand long service lives at high load and high temperatures. An optimized bearing arrangement of the rotor shaft is a fundamental prerequisite for this purpose.
Normally, plain bearing arrangements are used in exhaust-gas turbochargers. Due to the lower friction, plain bearing arrangements are increasingly being replaced by rolling bearings. It is typically the case in such an arrangement, that two rolling bearings are provided, which each have rolling elements. The rolling bearings are each assigned radially inner bearing elements, for example, inner bearing rings. These are each connected in positively locking or non-positively locking fashion to the shaft, or form an integral constituent part of the shaft. Furthermore, the rolling bearings are assigned in each case one outer bearing element, for example, a bearing sleeve element. Here, the two bearing sleeve elements may be arranged separately or else may be combined in a common bearing sleeve, or may be combined with one another in some other way. The inner bearing elements and the outer bearing elements each have running surfaces or raceways for the rolling elements, on which the rolling elements roll. The embodiment and arrangement of the running surfaces or raceways and the rolling elements are in this case selected or designed such that the rolling bearings can accommodate both radial bearing forces and axial bearing forces.
Here, with regard to the axial position and the axial play of the bearing arrangement, it is important that, for the purposes of an adjustment of the two rolling bearings, the axial spacing between the inner and outer bearing elements can be coordinated or adapted to the axial spacing between the two bearing sleeve elements, and the axial position of the rolling bearings in the accommodating bearing housing is defined.
Problems arise for example in the event of temperature changes that occur during operation, in the case of which a length expansion of the rotor shaft and/or of the outer bearing sleeve occurs, which can result in a loss of adjustment of the bearings (enlargement or decrease of the bearing play). The assembly of such a bearing arrangement in an accommodating bearing housing can also be cumbersome.
WO 2012/079881 A1 has disclosed a shaft bearing arrangement for an exhaust-gas turbocharger, which has two separate outer bearing rings which are coupled and are preloaded by means of a spring arrangement, where the rolling bearings are rendered free from play, and the position in relation to the rotor shaft is fixed.
The journal MTZ, 72nd volume, April 2011, pages 302-307, discloses a ball bearing unit for the rotor shaft of an exhaust-gas turbocharger. The ball bearing unit has two inner bearing shells composed of high-temperature-resistant bearing steel, which in the assembled state are pressed onto the rotor shaft of the exhaust-gas turbocharger. The balls of the ball bearing are composed of metal or ceramic and are held and guided separately from one another by means of a coated steel cage (may also be plastic). Here, the outer bearing sleeve elements are combined in a common bearing sleeve and are produced from a high-grade bearing steel.
Furthermore, the applicant's DE 10 2014 212 155.8 describes a bearing device for a shaft of an exhaust-gas turbocharger device of an internal combustion engine, which bearing device has at least two axially mutually spaced-apart rolling bearings with rolling elements. Here, in each of the rolling bearings, for the rolling elements, an inner running surface is arranged directly on the rotor shaft and an outer running surface is provided in a respectively associated outer bearing sleeve element. The two bearing sleeve elements are positioned coaxially with respect to one another by a common guide device and are thus combined to form a unit. The guide device converts a rotation of at least one of the bearing sleeve elements about the shaft longitudinal axis into a change in spacing of the two bearing sleeve elements, where the two bearing sleeve elements can be adjusted and/or fixed in the axial direction with regard to their spacing to one another and/or can be preloaded by a spring element.
The applicant's DE 10 2014 220 618.9 describes a bearing unit for the rotor, which has a rotor shaft, of an exhaust-gas turbocharger, which bearing unit has a bearing housing with a compressor side for connection to a compressor housing of the exhaust-gas turbocharger, a turbine side for connection to a turbine housing of the exhaust-gas turbocharger, and a cylindrical bearing-receiving opening with a bearing device received therein. The bearing-receiving opening is delimited in a radial direction by an inner shell of the bearing housing. The bearing device has two rolling bearings which are spaced apart from one another in an axial direction and of which one is arranged in the bearing housing at the turbine side and the other is arranged in the bearing housing at the compressor side. Each of the rolling bearings is assigned, for the rolling elements thereof, an inner bearing element with an inner running surface on the rotor shaft of the rotor and an outer bearing sleeve element with an outer running surface. The two outer bearing sleeve elements are positioned and fixed on a common guide concentrically and in an axial direction with respect to one another, in such a way that they form a bearing sleeve unit and it is ensured that the rolling bearings are free from play. The inner shell of the bearing housing has a step-like abutment shoulder. The outer diameters of the two bearing sleeve elements differ. The bearing sleeve element with the larger outer diameter bears in an axial direction against the abutment shoulder of the inner shell of the bearing housing.
DE 11 2010 001 779 T5 has disclosed a turbocharger which has a rolling bearing insert inserted into its bearing housing. The rolling bearing insert includes at least one inner running ring and at least one outer running ring and a row of rolling elements, each of which is in contact with one of the inner running rings and one of the outer running rings. In the turbocharger, an undesired rotation of the outer running ring in the mounted state is prevented using separate structural elements, for example using recesses and projections.